The study of the substrates of memory in higher vertebrates is one of the major problems of neurobiology. A simple and technically accessible experimental model is needed. Recent studies from this laboratory have demonstrated long-term adaptive plasticity, a form of memory, in the simplest, best-defined and most accessible pathway in the primate CNS, the two-neuron monosynaptic arc of the spinal stretch reflex (SSR). Monkeys can slowly change SSR amplitude without change in initial muscle length or alpha motoneuron tone, when reward is contingent on change. Change occurs over weeks and months, and persists for long periods. Thus SSR plasticity may prove a powerful model for studying the neuronal and synaptic substrates of memory in a primate. In addition, it may furnish a new therapeutic approach to spasticity and other manifestations of abnormal reflex function. The proposal's goal is to determine where function changes in the purely spinal, primarily monosynaptic, arc of the reflex. There are three major possibilities: change in muscle spindle function, change in function of the Ia synapse on the alpha motoneuron, and overall change in alpha motoneuron function. The project will study monkeys as SSR amplitude slowly changes from control size to one extreme (maximum or minimum) and then to the other extreme, in response to the reward contingency. It will determine which of several critical factors correlate with SSR amplitude. Measurement of stretch-induced afferent activity and the H reflex will distinguish between the muscle spindle and more central sites. Measurements of alpha motoneuron response to central and other peripheral inputs will evaluate the Ia synaptic regions vs. the alpha motoneuron in general. Study of single motor units will assess the homogeneity of alpha motoneuron behavior and detect changes within the population, such as alterations in recruitment. This work should define the location of the functional change subserving long-term adaptive plasticity in the SSR. This knowledge is essential for future detection and elucidation of the underlying persistent substrates and for development of therapeutic potential.